Electrical circuit manufacturing processes continue to advance facilitating the creation of ever smaller devices using ever smaller (finer) design rules. Referring to integrated circuit (IC) technology using an organic substrate, for example, fine line traces can be formed on the order of approximately 20 μm. Similarly, design rules specifying spacing can be on the order of approximately 20 μm. Correspondingly, the thickness of dielectric layers of an electrical circuit, whether implemented using printed circuit board (PCB) technology or IC technology, tends to be shallow. Again referring to IC technology, the thickness of a dielectric layer can be on the order of approximately 30 μm.
Given the feature sizes and fine design rules noted, implementation of a differential channel that meets a target of 100 Ohm differential impedance typically has a narrow trace width on the order of approximately 25 μm and a trace height of approximately 15-20 μm. A channel formed using one or more traces with dimensions as described can suffer from a variety of different types of loss that affect the ability of the channel to propagate a signal, particularly at high frequencies.
Channels as described can suffer from a variety of different types of loss including, but not limited to, dielectric loss incurred as a result of the dielectric material, loss incurred as a result of any roughness on the surface of the trace(s), and loss due to the skin effect. In general, the “skin effect” refers to the tendency of an alternating electric current, e.g., a high frequency signal, to distribute itself within a conductor so that the current density is largest at or near the surface of the conductor and decreases at greater depths from the surface of the conductor toward the center. The electric current flows mainly at or in the “skin” of the conductor (e.g., the trace). The “skin” can be defined as the portion of the conductor between the outer surface and a level within the conductor called the skin depth. The skin effect causes the effective resistance of a conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor.